The synthesis and release of phospholipids in the lungs is an important factor for fetal lung maturity. The phospholipids contribute to the stability of the peripheral airways by inhibiting the collapse of alveoli at low lung volume during expiration. They also act as an anti-edema factor by inhibiting the exudation of liquid from the pulmonary circulation into the airways. Lung surfactant can be recovered either from endobrachial lavage fluid or isolated from lung homogenate. The purified complex contains 85-95% lipid and 10-20% protein. About 90-95% w/w of the lipids are phospholipids with the remainder being cholesterol.
Lecithin (Phosphatidyl choline) has been recognized as the principal component accounting for 60-90% of the total phospholipids with sphingomyelin, phosphatidyl ethanolamine, phosphatidyl serine, lysolecithin, phosphatidyl inositol, and phosphatidyl glycerol comprising the remaining amount. The majority of these phospholipids are saturated, i. e., the two fatty acid residues contain no double bonds. The predominant fatty acid is palmitic acid (16:0). The dipalmitoyl lecithin derivative is the major surface active material, and it is acetone insoluble. It is well established that the gestational age of the fetus may be estimated by measuring the amounts of phosopholipids in the amniotic fluid. Louis Gluck, et al., American Journal Obstetrics & Gynecology, 109, 440-445, 1971, introduced the lecithin/sphingomyelin (L/S) ratio. This measurement was carried out by centrifugation of the amniotic fluid, cold acetone precipitation, thin layer chromatography, sulfuric acid charring of the separated phospholipids, and densitometry. This particular approach and its many modifications are the most widely used ones for assessing fetal lung maturity.
The quantitative measurement of phosphatidyl choline (lecithin) in amniotic fluid was described by S.G. Bhagwanani, et al., The Lancet 2, 66-67, 1972. The phospholipids were extracted with 2:1 chloroform-methanol, separated by thin layer chromatography, and following removal from the silica gel and digestion with perchloric acid, quantitated by phosphate measurement. These referred to methods, however, have the disadvantage of being time consuming and technically more difficult to carry out.